Articles made from thermoplastic resins are commonly utilized in the material-handling devices, electronic devices and business equipment, for example chip carriers, notebook computer enclosures and printer and copier components in contact with moving paper such as paper paths--and moving components themselves--such as ink-jet printer penholders. Electrostatic dissipation is an especially important issue within the electronic industry because of the inherently insulative nature of polymeric materials. Electrostatic dissipation (or discharge) is defined as a transfer of electrostatic charge between bodies at different potentials caused by direct contact or induced by an electrostatic field. As electronic devices become smaller and faster, their sensitivity to electrostatic dissipation (ESD) increases.
The US Department of Defense Handbook 263 (DOD-HDK-263) defines three categories of plastics for use in ESD protection: antistatic, static dissipating, and conductive. Characteristics of each type are listed in Table 1. Conductive fillers such as carbon fibers can be incorporated into polymeric materials to modify the electrical properties to achieve any of these three characteristics. In particular, carbon fibers facilitate dissipation of static charge and provide enhanced electromagnetic shielding. (See, for example, U.S. Pat. Nos. 4,559,164 and 5,004,561).
TABLE 1 Categories of Materials for ESD/EMI Protection Material Category Material Description Antistatic Will not generate a charge. Will not allow a charge to remain localized on part surface. Refers to a material's ability to resist triboelectric charge generation. Static Will not generate a charge. Dissipating Will not allow a charge to remain localized on part surface. Can safely bleed an electric charge to ground. Surface resistivity between 10.sup.5 and 10.sup.9 Ohm/Sq. Conductive Will not generate a charge. Will not allow a charge to remain localized on part surface. Can ground a charge quickly. Will shield parts from electromagnetic fields. Surface resistivity &lt;10.sup.5 Ohm/Sq.
Carbon fiber suppliers treat fiber surfaces to tailor the fibers for specific resin(s). Typical surface treatments used include air oxidation, anodic oxidation and wet oxidation. (J. M. Clark and D. R. Secrist, "Effect of Fiber Surface Treatment on the Properties of Carbon Fiber Reinforced Nylon Composites," Journal of Thermoplastic Composite Materials, Vol. 1, pp. 232-241 (1988); E. Fitzer and R. Weiss, "Effect of Surface Treatment and Sizing of C-Fibers on the Mechanical Properties of CFR Thermosetting and Thermoplastic Polymers," Carbon, Vol. 25, No. 4, pp. 455-467, (1987)). Binder is applied to these surface treated fibers and fibers are chopped. The purpose of the binder is to hold fiber bundles together (during handling and feeding). The binder is supposed to dissolve in the polymer matrix at high temperatures experienced during compounding performed in extruders. Utilization of binders is described in U.S. Pat. Nos. 4,781,947; 5,298,576; 5,641,572 and 5,639,807.
While carbon fibers have been utilized to improve electrical properties in thermoplastic resins, such fibers also contribute significantly to the cost of the articles, and can be detrimental to the impact strength and processability of the thermoplastic resin when used for injection molding. This problem is particularly significant in blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene copolymer (ABS). Such blends offer outstanding balance of mechanical properties, heat resistance, flow and cost. Performance of PC-ABS blend-based materials can be tailored for a particular application by varying the ratio of PC and ABS components. PC-ABS blends are often formulated with carbon fibers to impart high modulus and static dissipative properties. Due to the ease of processing and high flow, carbon fiber-filled PC-ABS compounds can be used to fill thin wall sections and complex parts in injection molding processes. However, one of the drawbacks of carbon fiber-filled PC-ABS is its low impact strength. Improvement in impact strength of carbon fiber-filled PC-ABS compositions, without affecting the modulus, static dissipative characteristics and cost would therefore be very useful.